1. Field of the Invention
The present invention relates to a method and an apparatus for detecting tone signals such as DTMF signals.
2. Description of the Related Art
In a voice communication using, for example, a fixed telephone or a portable telephone, a variety of services that utilize tone signals such as DTMF signals are available. Such services include, for example, reservations of tickets and messages in answering machines. For these services that require accurate detection of tone signals, methods such as a Fourier transformation and a filter bank using band-pass filter have been proposed.
The foregoing detecting methods, however, have been posing problems in that they require a huge volume of operation, or no satisfactory frequency resolution or time resolution can be obtained.
The present invention has been made with a view toward solving the problems described above, and an object thereof is to provide a method for detecting tone signals that permits high frequency resolution and high time resolution to be obtained by using a simple configuration.
To this end, according to the present invention, there is provided a method for detecting a tone signal, wherein a correlative value of an input signal obtained for each frame of a predetermined length is determined, an estimating expression is obtained that expresses an output power of a second order FIR band elimination filter that uses the correlative value of the input signal as an input and a frequency thereof as a parameter, a frequency at which a minimum value of the estimating expression is obtained is determined, an estimated gain of the FIR band elimination filter at the determined frequency is determined, and it is determined that the input signal is a tone signal if a value of the determined estimated gain exceeds a preset threshold value.